It is known that joints of the tripod constant speed type comprise a tripod element connected to a first shaft and provided with three trunnions on which spherical rollers are rotatively and slidably mounted and are received in runways formed in a tulip element connected to a second shaft. The tripod joints of the fixed type further comprise means for axially retaining the tripod element in the tulip element and comprising spherical domes carried by the tripod element and bearing against corresponding surfaces carried by the tulip element.
French patent No. 2,420,688 discloses such a tripod joint in which the bearing surfaces carried by the tulip element include in their central zones concave cavities the radii of curvature of which are slightly greater than those of the spherical surfaces rigid with the tripod element. Thus, as the tulip element is maintained axially against the tripod element by means of a muzzle constituting a resiliently yieldable fastener, this muzzle is in contact with the spherical surface of the tripod element through a spherical cavity whose radius is approximately equal to the radius of the tripod element and is formed in the planar surface of the muzzle.
As concerns the tulip element, it is in contact with the spherical bearing surface of a mushroom element which is slidably mounted in the tripod element by a spherical cavity whose radius is approximately equal to the radius of the spherical dome of the mushroom element and formed in the planar surface of the tulip element.
These small spherical cavities, also termed hereinafter "lunules", are aligned on the axis of the joint when the latter is not operating with the two shafts out of alignment. The lunules enable the pressures of the surfaces in contact under the load created by the mounting of a packing member whose thickness is slightly greater than the axial clearance measured between the confronting planar surfaces of the mushroom element and the tripod element to be considerably reduced.
The lunules remain substantially centered on the centre of the sphere of the tripod element for the most frequent operating angles of the joint, thus resulting in a decrease in the pressure of contact which is practically constant. On the other hand, when operating at a large angularity, an axial clearance appears between the mushroom element and the tripod element, this clearance being sufficient to result in the disappearance of the pre-stressing created by the packing element. It is then a spring, which is disposed inside the sphere of the tripod element and biases the mushroom element, which becomes operative for maintaining the tulip element axially.
It is with angularities between the relatively small angularity normally met with and the maximum angularity that the presence of the lunules is liable to present drawbacks. Indeed, when the joint operates at an angle, the tulip element becomes offcentre by a given angular value relative to the sphere of the tripod element and the mushroom element. Thus, the contact between the mushroom element and the tulip element and the contact between the tripod element and the muzzle no longer occur on the lunules but on the planar parts of the tulip element and the muzzle element, which has for result, for a given load, to multiply the Hertzian pressures by two in such a case.
Further, the offcentre produced causes the spherical domes of the tripod element and the mushroom element to leave the lunules and this results in an additional deflection of the muzzle element equal to twice the depth of the lunules. Consequently, there is a doubling of the load on the surfaces in contact.
Lastly, the effects due to the disengagement from the lunules are combined with those of the increase in the load and the initial Hertzian pressures are multiplied by three. Consequently, there are obtained conditions which encourage the appearance of noise and seizure of the spherical surfaces of the mushroom element and the tripod element produced by sliding under a critical load for lubrication.